This invention relates to the fabrication of composite materials, and, more specifically, to the consolidation of curved pieces of such materials.
Composite materials are hybrid materials formed by embedding reinforcement fibers, whiskers, or particles in a matrix which supports and protects the reinforcement. A variety of reinforcements are available, and carbon, graphite, glass, and Kevlar (trademark for an aromatic polyamide fiber) fibers in particular now enjoy widespread use in aerospace and other applications. Both metallic and nonmetallic matrices are available, and nonmetallics such as epoxies and polyimides are widely used for applications where the part is not exposed to high temperatures. Thus, for example, a composite material might consist of 60 volume percent of graphite fibers in a curable epoxy matrix.
In one common approach to the preparation of such materials, a manufacturer of the composite material prepares a "prepreg", which is a partially cured matrix having the proper amount of the reinforcement embedded therein. The reinforcement is commonly in the form of a unidirectional array or an interwoven, multidirectional fabric. The prepreg is provided in standard forms, as for example a tape 0.005 inches thick and 6 inches wide having a backing that permits its handling, or a fabric that is 0.013 inches thick. The prepreg is provided to the fabricator of parts, who then lays up a number of plies of the prepreg into the proper shapes and thicknesses as required by a design. The prepreg is the raw material that is used by the parts fabricator to build up the layered part.
After the prepreg tapes or fabric pieces are arranged in the proper stacked arrangement, the stacked plies must be consolidated to remove the porosity and voids that inevitably result from the layup process, and to complete the curing of the matrix. The consolidation is normally accomplished at an elevated temperature and with the application of external pressure to the part, so that the porosity and voids are closed before the matrix cures.
Where the part is, for example, a flat sheet, a piece of rigid tooling is placed against one side of the sheet and a flexible pressure membrane, such as a high temperature, gas impervious polyimide vacuum bag, is placed against the other side. A pressure is applied to the pressure membrane, either with a positive fluid (liquid or gas) pressure to the outside of the membrane, as for example about 200 pounds per square inch pressure, or with a vacuum applied to the space between the membrane and the composite material, so that the external atmospheric pressure supplies the consolidating pressure. Both positive pressure and vacuum consolidation are often used together for the best results.
Such existing techniques work well when the piece or body is flat or has a simple curvature. However, it is observed that, when complexly curved bodies are made by this consolidation approach, there may be irregularities such as bridging and folds on the concave-facing surface of the composite part, which can lead to reduced strength and premature failure of the composite material in that region.
There is a need for an improved approach to the consolidation of composite prepreg materials into final cured bodies, particularly where the bodies are complexly curved. The present invention fulfills this need, and further provides related advantages.